Image acquisition systems for visualization of various body organs and systems such as the cardiovascular system have won wide acceptance. For example, coronary cine or video angiograms are generated to discern the morphology and function of the coronary arterial vessels. However, other structures such as ribs and vertebra (background) diminish the ability to view the coronary arterial vessels. Consequently, various methods have been devised to decrease the background and enhance the areas of interest. For example, photographic or digital subtraction has been used to minimize or eliminate background. Background subtraction, as conventionally practiced, generally requires that two images be generated and recorded. The first image is usually generated without highlighting the vessels of interest with a contrast medium. The resultant picture or film is called a scout film. The second image is then generated and a second film obtained after the contrast medium has been injected into the vascular system so that the arterial vessels are highlighted. Common features of the first and second films are eliminated by subtracting the first film image from the second film image using well-known video (electronic), optical (photographic) or digital subtraction techniques.
In order to effect such subtraction, it is generally necessary to make two adjustments. The first adjustment is the registration of the first and second films by which corresponding elements of the two films are put into congruence. This is most simply done by translation and rotation of one of the images relative to the other. If there is any appreciable movement of the body between the two images, a more complex intraimage transformation may be required.
The second adjustment is the transformation of image density values into units of exposure. This is desirable so that the subtracted image will faithfully represent the vessel whether the background is dark or light.
Of course, the above techniques are useful in any of a number of different types of image acquisition systems such as cine, video, cut film, CAT, scan or ultrasound image acquisition systems.
Heretofore video, optical or digital subtraction techniques required that the image generator, such as the camera for a cine system or the image intensifier for a television or video system, and the subject remain stationary relative to each other during the entire time that the first and second films or digitized picture were generated. Indeed, one of the problems which has existed in prior subtraction techniques has been the necessity of assuring that the subject remains perfectly stationary and immobile during the entire time that the images are generated and stored. Even slight relative movement between the imaging device and the subject creates misregistration between the first film and the second film, thus resulting in blurring and a consequent lack of definition in the resultant subtracted image film.
This has also prevented extensive use of subtraction techniques in angiography because it has been general angiographic practice to pan the camera or other imaging device during the injection of the contrast medium. By this means, the flow of the contrast medium can be observed through the arterial bed without sacrificing details. When it is desired to use this panning process substantial amounts of contrast medium must be injected into the subject to get the desired degree of definition of the arteries without an unacceptacle level of background interference. This can be dangerous to the subject.
A procedure recently adopted to alleviate the danger of high contrast medium concentrations has been to take a number of pictures of a subject with a decreased amount of contrast medium and thereafter register and average the multiple pictures to provide the degree of definition desired. Such a technique was the subject of our patent application entitled "Image Averaging for Angiography by Registration in Combination with Serial Images," U.S. Pat. No. 4,263,916, of which this application is a continuation-in-part. This technique is very useful but was particularly directed to multiple pictures of the subject at a fixed position relative to the imaging device. Such is not the case when the imaging device pans the subject.
Of course, various other techniques are known for combining multiple images to form a composite image. However, such techniques have not been heretofore used in conjunction with image subtraction where there is relative movement between the imaging device and the subject during a panning procedure.
The present invention provides for such subtraction even though there is relative movement between the imaging device and the subject by providing apparatus whereby the motion between the imaging device and the subject can be sensed, stored and later duplicated so that frame-by-frame background subtraction can be done. The present invention is applicable in the fields of radiography, angiography, digital angiography, ultrasound scanning, nuclear imaging, CAT scanning or any other type of imaging.
More specifically, the present invention comprises apparatus and methods for sensing and recording relative position parameters between the imaging device and the subject over a period of time to describe completely the motion geometry between the imaging device and the subject along a movement path selected by the operator. The positional parameters are then used to generate commands which drive a servo motor system whereby all or part of the imaging device is moved to duplicate the operator's original movement path. Reference to the imaging device herein includes the support table for the subject, an illumination source and an illumination receiver and movement of the imaging device means movement of one or more components of the imaging device to effect relative movement between the subject and the illumination receiver.
In accordance with the invention, a first sequence of images is recorded to define a first motion picture as movement occurs along the movement path under operator control and a second sequence of images is recorded to define a second motion picture as subsequent movement occurs along the movement path under automatic control.
The first and second motion pictures may be made immediately after each other in which case the first motion picture is made while a contrast medium is in the arterial bed of the subject and the second motion picture is made after the contrast medium has been substantially diluted or otherwise purged from the arterial bed.
Alternatively, the first and second motion pictures may be made with a substantial time interval of even several years therebetween so that the subtracted image motion picture will be a difference motion picture highlighting the changes which occurred during the time interval between the first and second motion pictures. With respect to this latter case, if a contrast medium is to be used, as is generally desired in radiographic imaging, then both the first and second motion pictures should be generated while the contrast medium is in the arterial bed. Otherwise, neither the first nor the second motion picture should be made while there is contrast medium in the arterial bed.
The individual frames of both the first and second motion pictures, whether or not taken with a contrast medium present are then associated with a particular position along the movement path so that every first motion picture frame is associated with a corresponding second motion picture frame by aligning the frames according to correspondence of the associated position information. While there would necessarily be some non-congruence due, for example, to subject respiration, movement and cardiac cycle, by techniques such as timing or multiple or very slow second motion picture runs, it is possible to get or select second motion picture frames which are in good registration with the first motion picture frames. Translation, rotation and more complex transformations may also be used in accordance with the invention to insure satisfactory infraframe registration. Similarly, recording or image intensifier information and step wedge information enables suitable nonlinear subtraction to be carried out.